The present invention relates to a processing machine, in particular to a processing center, with a spindle, a tool clamping device, a converter unit with a spindle motor, at least one oscillation sensor and a balancing device.
An imbalance exists if the rotation axis is offset relative to the mass center of gravity of a rotating mass. The rotational imbalance causes the rotational body to generate a centrifugal force. This force is a dynamic function of the frequency of the rotation speed. When the rotation speed doubles, the centrifugal force increases fourfold and can overload the spindle bearing(s). The spindle bearings transmit this dynamic force to the machine frame, producing a more or less pronounced dynamic excursion depending on the stiffness of the machine frame. The amplitude of this dynamic excursion is proportional to the imbalance, with the phase corresponding to the imbalance angle.
The dynamic excursion is measured by an oscillation or vibration sensor. The oscillation sensor can be, for example, a piezoelectric sensor which converts the acceleration into electrical signals. The excursion is determined by a frequency analysis through multiple integration and filtering of the measured signal and is displayed, for example, in units of □m. The phase of the signal can also be measured in addition to the amplitude of the imbalance and used to determine and display the angular position of the imbalance.
By narrow-band filtering the measured signal with a bandpass close to the rotation frequency, oscillation frequencies produced by other excitation sources can be suppressed to produce more exact measurement values.
The imbalance can be best visualized by displaying the oscillation excursion. Conventionally, the oscillation speed is used as a representative measurement unit in oscillation measurements. A simple mathematical relationship is obtained between the oscillation excursion and the oscillation frequency of the oscillation produced by the imbalance.
However, even with great mathematical effort, the relationship between oscillation excursion/oscillation speed and the imbalance can only be calculated approximately. This depends strongly on the measurement location, the machine construction and the materials employed for the spindle, the bearing, the machine frame and the foundation. For this reason, a defined imbalance is applied at a certain radius and the relationship between imbalance and oscillation excursion and/or oscillation speed is established in during a single transient. This process is referred to as calibration. After this setup process, a balancing electronic can calculate the magnitude and the angle position of the balance weights at each subsequent balancing process.
High-speed processing, also referred to as High Speed Cutting (HSC), is a relatively new technology with encouraging performance features and advantages for the metal-cutting industry. Machine spindle rotation speeds in excess of 12,000 min−1 to 50,000 min−1 are advantageous for processing as they significantly increase the quantity of removed material and hence the productivity which in turn enables new cost-effective manufacturing processes. The reduced cutting forces can also improve the surface quality. With increasing spindle rotation speed, the centrifugal forces produced by the mass asymmetries (imbalances) increase with the square of the spindle rotation speed. These large forces can destroy the tools and/or the spindle and produce workpieces with poor surface quality. These imbalances can be eliminated with commercially available automatic balancing systems, which typically include a balancing device and a display and control device with balancing electronics.
These conventional balancing devices are electromechanical units. Two electric motors with special gear units are combined in each balancing device. These assemblies are used to position two independently supported balance masses which then balance the imbalances. The balance masses can be adjusted when the balancing device is at rest as well as during the rotation (at operating rotation speed) of the balancing device. To position the two independently supported balancing masses in the balancing device, the evaluation and control device computes from the measurement results of the oscillation sensors two angular position values, whereby the imbalance is compensated by shifting the masses to the computed angular position values. The actuating motors then move automatically to these positions. The evaluation and control device supplies the required electric power for the motors. The power is transmitted conventionally with brush rings or contactless. Balancing devices of this type can be flanged on or built into the spindle. The magnitude of the imbalance and the available space on the spindle of the processing machine determine the type size of the balancing device to be used.
As mentioned above, the balancing device is used only to adjust the balancing masses so as to eliminate the imbalance. This also requires a display and control device and at least one oscillation sensor. The oscillation sensors are connected with a measurement signal input of the display and control device. The balancing device and a rotation speed sensor of the spindle to be balanced is connected with additional terminals of the display and control device.
The conventional display and control device is also available as a rack-mounted unit or as a tabletop unit. A rack-mounted unit is preferred in processing machines because it can be installed in a control cabinet that forms a part of the processing machine.
If this automatic balancing system is used in a CNC machine, then a balancing module with a CNC connection can be used instead of a display and control device. Also required are a machine interface module as well as a single channel or a dual channel monitor module. A preamplifier may also be necessary. The automatic balancing system with a CNC connection requires significantly more complex and expensive wiring than the aforementioned embodiment. In addition to the wiring expense, the control cabinets of the processing machine must also have sufficient space for the display and control device and/or for the modules.
It would therefore be desirable and advantageous to provide a processing machine with an automatic balancing system, which obviates prior art shortcomings and is able to specifically reduce wiring complexity and space requirements.